We present a new technique for probing the properties of quantum fluids inrestricted geometries. We have confined liquid 4He within microfluidic devicesformed from glass wafers, in which one dimension is on the micro scale. Usingan ultrasonic analog to Fabry-Perot interferometry, we have measured thefirst-sound of the confined liquid 4He, which can be a probe of criticalbehavior near the lambda point ($T_{\lambda}$). All thermodynamic properties ofliquid $^4$He can be derived from first-sound and heat capacity measurements,and although quite a bit of experimental work has been done on the latter, nomeasurement of first-sound has been reported for a precisely confined geometrysmaller than a few tens of micrometers. In this work, we report measurements ofisobaric first sound in liquid $^4$He confined in cavities as small as ~ 5${\mu}$m. Our experimental set-up allows us to pressurize the liquid up to ~ 25bar without causing deformation of the confined geometry, a pressure which isabout 4 times larger than previously reported with similar microfluidicdevices. Our preliminary results indicate that one can possibly observefinite-size effects and verify scaling laws, by using similar devices withsmaller confinement.
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